BE Civil Engineering (IOE, TU) Engineering Chemistry (IOE, SH 403) Question Paper 2078 Nepal

Definitions (2 marks)

• Electrode potential: the tendency of a metal electrode to lose or gain electrons when in contact with a solution of its own ions; it is the potential difference developed at the metal–solution interface.
• Standard electrode potential ($E^{\circ}$): the electrode potential measured under standard conditions (1 M ion activity, 1 bar pressure for gases, 25 °C) with respect to the Standard Hydrogen Electrode (SHE), whose potential is taken as 0 V.

Nernst equation (2 marks)

For an electrode reaction $\text{M}^{n+} + n e^- \rightarrow \text{M}$:

$$E = E^{\circ} - \frac{RT}{nF}\ln\frac{1}{[\text{M}^{n+}]}$$

At 25 °C this reduces to $E = E^{\circ} - \dfrac{0.0591}{n}\log\dfrac{1}{[\text{M}^{n+}]}$, where $R$ = gas constant (8.314 J mol⁻¹ K⁻¹), $T$ = absolute temperature (K), $n$ = number of electrons transferred, $F$ = Faraday constant (96485 C mol⁻¹).

Cell EMF calculation (6 marks)

Standard cell EMF:

$$E^{\circ}_{\text{cell}} = E^{\circ}_{\text{cathode}} - E^{\circ}_{\text{anode}} = (+0.34) - (-0.76) = +1.10\ \text{V}$$

Cell reaction: $\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}$, with $n = 2$.

Reaction quotient:

$$Q = \frac{[\text{Zn}^{2+}]}{[\text{Cu}^{2+}]} = \frac{0.010}{0.50} = 0.020$$

Apply the Nernst equation:

$$E_{\text{cell}} = E^{\circ}_{\text{cell}} - \frac{0.0591}{n}\log Q = 1.10 - \frac{0.0591}{2}\log(0.020)$$

$\log(0.020) = -1.699$, so:

$$E_{\text{cell}} = 1.10 - (0.02955)(-1.699) = 1.10 + 0.0502 = 1.1502\ \text{V}$$

Final answer: $E_{\text{cell}} = \mathbf{+1.15\ V}$. Since $E_{\text{cell}} > 0$ (equivalently $\Delta G = -nFE < 0$), the cell reaction is spontaneous.

Electrochemical theory of corrosion (4 marks)

Wet corrosion occurs when a metal surface in contact with moisture and oxygen behaves like many tiny short-circuited galvanic cells. Differences in composition, stress, or oxygen concentration set up anodic and cathodic regions on the same metal surface; an electrolyte (moisture containing dissolved salts/CO₂) completes the circuit.

For iron exposed to moist air:

• Anodic region (oxidation):

$$\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$$

• Cathodic region (oxygen-absorption type, neutral/aerated water):

$$\tfrac{1}{2}\,\text{O}_2 + \text{H}_2\text{O} + 2e^- \rightarrow 2\,\text{OH}^-$$

The Fe²⁺ ions migrate and meet OH⁻ to form ferrous hydroxide, which is further oxidised by atmospheric oxygen to hydrated ferric oxide (rust):

$$\text{Fe}^{2+} + 2\,\text{OH}^- \rightarrow \text{Fe(OH)}_2$$ $$4\,\text{Fe(OH)}_2 + \text{O}_2 + 2\,\text{H}_2\text{O} \rightarrow 4\,\text{Fe(OH)}_3$$

Overall (simplified):

$$4\,\text{Fe} + 3\,\text{O}_2 + 2x\,\text{H}_2\text{O} \rightarrow 2\,\text{Fe}_2\text{O}_3\!\cdot\! x\text{H}_2\text{O} \;(\text{rust})$$

(i) Cathodic protection by sacrificial anode (2.5 marks)

A more active (more easily oxidised) metal such as Mg, Zn or Al is electrically connected to the iron structure (e.g., buried pipeline, ship hull). The active metal becomes the anode and corrodes preferentially, forcing the entire iron structure to act as cathode, so it is protected. The sacrificial anode is consumed and replaced periodically.

  Mg block ---- wire ---- Steel pipeline (cathode, protected)
   (anode, corrodes)         buried in soil (electrolyte)

(ii) Galvanizing (2.5 marks)

Galvanizing is coating iron/steel with a layer of zinc (usually by hot dipping). Zinc gives barrier protection (physically excludes air and moisture) and, importantly, sacrificial protection: even if the coating is scratched and iron is exposed, zinc (being more active) corrodes preferentially and protects the iron.

Difference (1 mark)

Galvanizing applies a continuous physical coating of zinc directly on the iron (coating-based, distributed). Sacrificial-anode cathodic protection uses a separate bulk anode connected electrically to an otherwise uncoated structure; the anode need not touch the whole surface and is periodically replaced. Galvanizing suits small fabricated items; sacrificial anodes suit large structures like pipelines and ship hulls.

Polymerization (1 mark)

Polymerization is the chemical process in which a large number of small molecules (monomers) join together through covalent bonds to form a high-molecular-weight macromolecule (polymer).

Addition vs condensation polymerization (3 marks)

Thermoplastic vs thermosetting (4 marks)

Degree of polymerization (2 marks)

Molar mass of the ethylene repeat unit $\text{C}_2\text{H}_4$:

$$M_0 = 2(12) + 4(1) = 28\ \text{g/mol}$$

Degree of polymerization:

$$\text{DP} = \frac{\bar{M}_n}{M_0} = \frac{42{,}000}{28} = 1500$$

Final answer: degree of polymerization $= \mathbf{1500}$.

Hardness and its types (3 marks)

Hardness is the property of water that prevents lathering with soap, caused by dissolved salts of calcium and magnesium (and other multivalent cations).

• Temporary (carbonate) hardness: due to bicarbonates of Ca²⁺ and Mg²⁺, e.g., Ca(HCO₃)₂, Mg(HCO₃)₂. Removed by boiling.
• Permanent (non-carbonate) hardness: due to chlorides and sulphates of Ca²⁺ and Mg²⁺, e.g., CaCl₂, CaSO₄, MgCl₂, MgSO₄. Not removed by boiling.

Total hardness by EDTA (4 marks)

EDTA reacts with Ca²⁺/Mg²⁺ in a 1:1 mole ratio. Moles of EDTA used:

$$n_{\text{EDTA}} = M \times V = 0.010\ \text{mol/L} \times 0.0180\ \text{L} = 1.80\times 10^{-4}\ \text{mol}$$

This equals moles of hardness ions (expressed as CaCO₃). Mass of CaCO₃ equivalent ($M_{\text{CaCO}_3} = 100\ \text{g/mol}$):

$$m = 1.80\times 10^{-4}\ \text{mol} \times 100\ \text{g/mol} = 1.80\times 10^{-2}\ \text{g} = 18.0\ \text{mg}$$

This 18.0 mg is present in 100 mL of water. Convert to ppm (mg per litre, since 1 L of water ≈ 1 kg):

$$\text{Hardness} = \frac{18.0\ \text{mg}}{100\ \text{mL}} \times 1000\ \frac{\text{mL}}{\text{L}} = 180\ \text{mg/L}$$

Final answer: total hardness $= \mathbf{180\ ppm\ as\ CaCO_3}$.

Lime–soda process (3 marks)

Lime [Ca(OH)₂] and soda ash [Na₂CO₃] are added to precipitate Ca²⁺ and Mg²⁺ as insoluble CaCO₃ and Mg(OH)₂, which are then filtered out.

• Temporary hardness (lime removes bicarbonate):

$$\text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \rightarrow 2\,\text{CaCO}_3\!\downarrow + 2\,\text{H}_2\text{O}$$

• Permanent hardness (soda removes chloride/sulphate):

$$\text{CaCl}_2 + \text{Na}_2\text{CO}_3 \rightarrow \text{CaCO}_3\!\downarrow + 2\,\text{NaCl}$$

Lime supplies OH⁻/CO₃²⁻ to handle carbonate hardness and Mg²⁺; soda supplies extra CO₃²⁻ to remove permanent calcium hardness.

Definitions (3 marks)

• Calorific value: the amount of heat liberated by the complete combustion of a unit mass (or volume) of a fuel. Units: kcal/kg (solids/liquids) or kcal/m³ (gases).
• Gross / Higher Calorific Value (GCV/HCV): total heat released when unit mass of fuel is completely burnt and the combustion products are cooled back to the original (room) temperature, so that the water vapour formed is condensed and its latent heat is recovered.
• Net / Lower Calorific Value (NCV/LCV): heat released when the combustion products are not cooled, i.e., the water escapes as vapour and its latent heat of vaporisation is not recovered. Thus NCV = GCV − latent heat of the water formed.

Dulong's formula (1 mark)

$$\text{GCV} = \frac{1}{100}\left[ 8080\,C + 34500\left(H - \frac{O}{8}\right) + 2240\,S \right]\ \text{kcal/kg}$$

where C, H, O, S are percentages by mass.

GCV calculation (3 marks)

Available (net) hydrogen $= H - \dfrac{O}{8} = 6 - \dfrac{5}{8} = 6 - 0.625 = 5.375$

$$\text{GCV} = \frac{1}{100}\big[ 8080(80) + 34500(5.375) + 2240(1) \big]$$

• $8080 \times 80 = 646{,}400$
• $34500 \times 5.375 = 185{,}437.5$
• $2240 \times 1 = 2{,}240$
• Sum $= 646{,}400 + 185{,}437.5 + 2{,}240 = 834{,}077.5$

$$\text{GCV} = \frac{834{,}077.5}{100} = 8340.775\ \text{kcal/kg}$$

Final GCV $= \mathbf{8340.78\ kcal/kg}$ (≈ 8340.8 kcal/kg).

NCV calculation (3 marks)

Mass of water formed per kg fuel $= 0.09 \times H = 0.09 \times 6 = 0.54\ \text{kg}$

Latent heat correction $= 0.54 \times 587 = 316.98\ \text{kcal/kg}$

$$\text{NCV} = \text{GCV} - 316.98 = 8340.775 - 316.98 = 8023.795\ \text{kcal/kg}$$

Final NCV $= \mathbf{8023.80\ kcal/kg}$ (≈ 8023.8 kcal/kg).

Catalyst (1 mark)

A catalyst is a substance that alters (usually increases) the rate of a chemical reaction without itself being consumed permanently, by providing an alternative reaction path of lower activation energy.

Homogeneous vs heterogeneous catalysis (2.5 marks)

Characteristics of a catalyst (1.5 marks)

1. A small quantity of catalyst is sufficient to catalyse a large amount of reactant.
2. It is chemically unchanged in mass and composition at the end of the reaction (though it may change physically).
3. It does not initiate a reaction; it only changes the rate and does not alter the position of equilibrium (it speeds up forward and reverse reactions equally). It is usually specific in action and can be poisoned.

Major (Bogue) compounds of Portland cement (2 marks)

Setting and hardening (2 marks)

When water is added to cement, the compounds undergo hydration and hydrolysis, forming gel and crystalline products:

• C₃A reacts first and very fast, giving early (initial) set.
• C₃S hydrates within hours/days giving calcium silicate hydrate (C–S–H gel) and Ca(OH)₂, responsible for early strength.
• C₂S hydrates slowly over weeks/months giving more C–S–H gel, responsible for ultimate (long-term) strength.

Setting is the initial stiffening (loss of plasticity) of the paste; hardening is the subsequent gradual development of strength as the C–S–H gel grows and interlocks into a rigid mass.

$$2(3\text{CaO}\cdot\text{SiO}_2) + 6\text{H}_2\text{O} \rightarrow 3\text{CaO}\cdot 2\text{SiO}_2\cdot 3\text{H}_2\text{O} + 3\,\text{Ca(OH)}_2$$

Role of gypsum (1 mark)

Gypsum ($\text{CaSO}_4\cdot 2\text{H}_2\text{O}$, about 2–3%) is added to retard the flash (instant) setting caused by the very rapid hydration of C₃A. It reacts with C₃A to form ettringite, slowing the set and giving enough working time.

Paint (1 mark)

A paint is a mechanical dispersion (suspension) of one or more finely divided pigments in a liquid medium (vehicle), applied as a thin coating that dries to an opaque, adherent, protective and decorative film over a surface.

Constituents and their functions (3 marks)

(Any four constituents with correct functions earn full marks.)

Paint vs varnish (1 mark)

A paint contains a pigment, so it gives an opaque, coloured film. A varnish is a homogeneous solution of a resin (natural or synthetic) in a drying oil and/or volatile solvent without any pigment; it dries to a transparent, glossy film used to give a protective, decorative finish that shows the natural grain of the underlying surface (e.g., wood).

Explosive (1 mark)

An explosive is a substance (or mixture) that, on receiving a suitable stimulus (heat, shock, friction or spark), undergoes a very rapid, self-sustaining exothermic decomposition producing a large volume of hot gases and a sudden release of energy, generating a destructive shock/pressure wave.

Classification by performance (2 marks)

1. Primary (initiating) explosives — extremely sensitive; detonate by a small stimulus and are used to set off other explosives. e.g., lead azide, mercury fulminate.
2. Secondary (high) explosives — relatively insensitive, need a primary explosive to detonate; very powerful. e.g., TNT (trinitrotoluene), RDX.
3. Low explosives (propellants) — undergo rapid combustion (deflagration) rather than detonation; used as propellants. e.g., gunpowder (black powder), nitrocellulose.

Characteristics of a good explosive (1.5 marks)

1. It should be chemically stable and safe to store/handle, yet detonate reliably when initiated.
2. It should produce a large volume of gas with a high heat of explosion (high brisance/power) in a very short time.

(Other acceptable points: insensitive to accidental shock/friction, easy and cheap to manufacture, suitable detonation velocity.)

Why TNT is widely used (0.5 mark)

TNT is favoured militarily because it is chemically stable and insensitive to shock/friction (safe to handle and store), has a convenient low melting point (~81 °C) so it can be safely melt-cast into shells, yet it is powerful and reliable when detonated by a booster, and it does not react with metals.

Definitions and units (2 marks)

• Specific conductance (conductivity, $\kappa$): the conductance of a solution held between two electrodes of unit area (1 cm²) placed unit distance (1 cm) apart, i.e., the conductance of 1 cm³ of the solution. Unit: $\text{S cm}^{-1}$ (ohm⁻¹ cm⁻¹).
• Molar conductance ($\Lambda_m$): the conducting power of all the ions produced by dissolving 1 mole of electrolyte in a given volume of solution. Unit: $\text{S cm}^2\ \text{mol}^{-1}$.

Relation: $\Lambda_m = \dfrac{1000\,\kappa}{C}$, with $C$ in mol/L.

(i) Specific conductance (1.5 marks)

Conductance $G = \dfrac{1}{R} = \dfrac{1}{250} = 4.0\times 10^{-3}\ \text{S}$

$$\kappa = G \times \text{cell constant} = (4.0\times 10^{-3}\ \text{S})(1.25\ \text{cm}^{-1})$$ $$\kappa = 5.0\times 10^{-3}\ \text{S cm}^{-1}$$

Specific conductance $= \mathbf{5.0\times 10^{-3}\ S\,cm^{-1}}$.

(ii) Molar conductance (1.5 marks)

$$\Lambda_m = \frac{1000\,\kappa}{C} = \frac{1000\ (\text{cm}^3/\text{L}) \times 5.0\times 10^{-3}\ \text{S cm}^{-1}}{0.020\ \text{mol/L}}$$ $$\Lambda_m = \frac{5.0}{0.020} = 250\ \text{S cm}^2\ \text{mol}^{-1}$$

Molar conductance $= \mathbf{250\ S\,cm^2\,mol^{-1}}$.

Natural rubber (1.5 marks)

Natural rubber is a high-molecular-weight linear polymer of isoprene (2-methyl-1,3-butadiene), i.e., cis-1,4-polyisoprene, obtained as latex from the rubber tree (Hevea brasiliensis).

$$n\,\text{CH}_2{=}\text{C(CH}_3){-}\text{CH}{=}\text{CH}_2 \longrightarrow {-}[\text{CH}_2{-}\text{C(CH}_3){=}\text{CH}{-}\text{CH}_2]_n{-}$$

Drawbacks of raw natural rubber (1.5 marks)

1. Soft and sticky when hot, brittle and hard when cold (poor temperature range).
2. Low tensile strength, low elasticity (large permanent set after stretching) and low wear/abrasion resistance.
3. Swells and dissolves in organic solvents; is attacked by oxidising agents and oxygen/ozone (perishes).

Vulcanization (2 marks)

Vulcanization is the process of heating raw rubber with sulphur (about 3–5% for soft rubber, more for hard rubber) at 100–140 °C, optionally with accelerators. Sulphur atoms form cross-links (sulphur bridges) between adjacent polyisoprene chains at the C=C double bonds.

   ...chain 1...  C=C ...
                  |
                  S  (sulphur cross-link)
                  |
   ...chain 2...  C=C ...

The cross-linking converts the soft, plastic mass into a stronger, elastic three-dimensional network. As a result, vulcanized rubber has: higher tensile strength and elasticity, better abrasion/wear resistance, a wider useful temperature range (less tacky when hot, less brittle when cold), greater resistance to solvents and oxidation, and improved durability.